In the drilling of boreholes through underground formations for the purposes of locating and producing oil and gas, and for the purposes of mining and production of steam energy through thermal wells, the most common type of drilling apparatus used today is the tri-cone rolling cutter drill bit. This bit generally comprises a central body section having three legs extending downwardly therefrom. Each leg has an inwardly projecting bearing journal upon which is rotatably mounted a frustoconical cutter. Generally, the most prevalent type of cutting structure utilized in the tri-cone bit is the tungsten carbide insert cutting structure. Tungsten carbide cutting elements are press-fit in holes drilled in the frustoconical cutters and protrude outwardly to provide a digging, crushing and gouging action on the bottom of the borehole as the bit is rotated.
The tungsten carbide insert bit has generally been known and used for approximately the last 30 years. For the first 20 years (1950 to about 1970), those in the art felt that the cutting structure of the insert bit should be of the nonoffset or "true rolling cone" type. The offset, which is defined as the amount by which the rotational axes of the rolling cutters is offset from the rotational axis of the main bit, was a feature found in milled tooth bits but believed to be detrimental to insert bits because of the breakage problem in the tungsten carbide inserts when the additional drag forces were introduced through the use of offset.
In February, 1970, a new bit design was patented by P. W. Schumacher, Jr. (U.S. Pat. No. 3,495,668) in which, for the first time, an insert bit successfully incorporated offset axis cutters to achieve greater gouging and scraping action in the borehole. A subsequent patent, U.S. Pat. No. 3,696,876, issued to Ott in October, 1972, also disclosed a similar invention wherein offset axis cutting elements were incorporated into an insert bit.
Drilling bits incorporating the novel combination of offset cutters and tungsten carbide inserts were successfully introduced by the assignee of the present invention, Reed Rock Bit Company, in 1970, and have become the most prevalent type of drill bits in the drilling industry over the past ten years. This second generation of drill bits utilizing offset axes and tungsten carbide inserts are particularly advantageous in soft to medium-soft formations by reason of their introduction of a gouging and scraping action which enhances the drilling efficiency and rate of penetration of the bit in these formations. The amount of offset utilized in these bits ranges on the order of from about 1/64 to about 1/32 inch offset per inch of drill bit diameter. For instance, a 77/8 inch bit having offset would have from 1/8 inch to 1/4 inch total offset in the cutters.
Conventional drilling bits currently on the market are limited in the amount of offset introduced into the cutters to about 1/32 inch of offset per inch of diameter. Thus, the maximum amount of offset utilized in these soft formation bits currently runs about 1/4 inch in a 77/8 inch diameter bit. During this ten year period when offset axis insert bits have been made commercially successful, those skilled in the art of drill bit technology generally have followed the principle that any additional offset in the cutters above about 1/32 inch per inch of bit diameter would not add any significant efficiency or increased drilling rate to the bit to justify the increased breakage that such increased offset would introduce. In fact, drilling tests conducted utilizing insert bits with offset somewhat greater than 1/32 inch per inch of bit diameter have indicated insignificant gains in rate of penetration, but larger incidences of insert breakage. Thus, those skilled in the art have restricted their insert bit designs to having an offset range of from zero to 1/32 inch per inch of bit diameter.
The present invention utilizes a unique insert bit design having great amounts of offset in the cutting structure far exceeding those ranges utilized in conventional offset-axis insert bits. It was found by this inventor that when offset equal to or greater than 1/16 inch per inch of bit diameter was introduced into a tri-cone insert bit, that greatly significant increases in rate of penetration and bit performance can be obtained. For some reason unknown to the inventor, the penetration rate and drilling efficiency of an offset insert bit does not increase significantly from about 1/32 inch offset per inch of bit diameter (upper range of conventional insert offset bits) up to about 1/16 inch offset per inch of bit diameter. It was discovered though that beginning at about 1/16 inch offset per inch of bit diameter a significant jump in the rate of penetration and drilling efficiency was noted.
The use of large amounts of offset in milled-tooth rolling cutter drill bits may not in itself be a novel concept. For instance, see U.S. Pat. no. 1,388,456 to H. W. Fletcher, dated Aug. 23, 1921, in which a two-cone rolling cutter drill bit having milled tooth cutters apparently incorporated a large amount of offset in the two cutters. The patent discloses no specific amount of offset to be utilized and, as far as this inventor is aware, no commercial embodiment of the Fletcher design ever became successful. The conventional milled tooth drill bits which have been available for the last 40 years have generally utilized offset in the range of 1/64 to 1/32 inch per inch of bit diameter and have been tri-cone bits. It was not until 1970, and the issuance of the Schumacher patent, that the industry was introduced to the use of insert type bits utilizing the offset already present in milled tooth bits. The reason that the high offset cutters were not thought practical was that increases in offset above the 1/32 inch limit previously mentioned would gain very little in cutting efficiency, but increased the amount of breakage of tungsten carbide inserts in the insert type bits. Also, increasing the offset necessarily requires reducing the size of the cutter cones to prevent interference between the inserts on adjacent cones. Smaller cones mean smaller bearing areas and/or thinner cone shells, both of which add to earlier bit failure. Also, greater offset means less efficient intermeshing of inserts on adjacent cones which in turn reduces the amount of self-cleaning of the inserts and increases "balling-up".
Conventional jetting systems are generally made up of two different types. The oldest type is the regular drilling fluid system where large, relatively unrestricted fluid openings are provided in the bit body directly above the cutter cones to allow a low pressure flow of the drilling fluid to fall on the cones and move around the cones to the bottom of the borehole. By necessity, this is a low-volume, low-velocity flow since the fluid stream impinges directly upon the cutter face, and abrasion of the cones is a serious problem under these circumstances. The second type of conventional bit fluid system comprises the "jet" bits. In a jet bit a high pressure jet of fluid is generated from the bit body directly against the formation face without impinging on any cutting elements or any portion of the bit. In some instances, the so-called jet bits have fluid nozzles extending from the bit bodies all the way downward to a point only a fraction of an inch above the formation face to maximize hydraulic energy of the fluid stream impinging the formation face. The conventional jet bits do not emit fluid against any cutting elements because of the adverse effect of erosion from the high-pressure drilling fluid. The present invention differs from these two conventional types in that it uses a directed jet spray which impinges directly upon the cutter inserts.
The present invention discloses an insert type bit, as opposed to a milled tooth bit, which insert bit utilizes rolling cone cutting elements rotatably mounted on lugs having rotational axes with large offset from the rotational axis of the drill bit. The amount of offset ranges between 1/16 and 1/8 inch per inch of bit diameter. The resulting invention produces greatly increased rates of penetration and drilling efficiency when utilized in soft to medium-soft formations. It should be noted that the present invention, when embodied in a tri-cone oilwell drilling bit, suffers a greater amount of erosion and breakage of the hard metal cutting inserts in the cones, but the total gain in drilling efficiency and rate of penetration far offsets the increased wear and breakage of the cutting elements.
In addition to the aforementioned unique drill bit construction, the present invention also embodies a new and unique nozzle jetting system for delivering drilling fluid to the cutting elements and the face of the formation as it is being drilled. This jetting system utilizes directed nozzles which create a spray of pressurized drilling fluid and directs this spray across the protruding tungsten carbide inserts and against the formation face. The new jetting system provides a dual function of cleaning material from the inserts and also sweeping the cuttings fromm the borehole face. This system is particularly advantageous when drilling through those certain types of formations which, because of their softness or ductility, become very plastic during drilling operations, and tend to "ball up" in the spaces between the inserts on the cutters. This "balling up" greatly reduces the rate of penetration and the cutting efficiency of drill bits when penetrating such plastic formations. The new jetting system provides a plurality of fluid jets directed at preselected angles to spray drilling fluid across the inserts without impinging the cutter cone surfaces, with the spray also being directed against the formation face to further flush and clean the cuttings as they are gouged and scraped out of the formation.